Chemical mechanical polishing (CMP) is one method of providing a planarized substrate surface. Such substrates are used in the manufacture of integrated circuit devices. CMP may be used to planarize raw substrates or to completely or partially remove a bulk deposited layer, but more is commonly used to planarize a surface by partially removing layers which have been deposited over non-planar features formed in or on a subjacent layer. A typical CMP apparatus employs a rotating polishing surface, such as a consumable polishing pad, against which the surface of the substrate being polished is placed. The CMP apparatus also includes a carrier which secures the substrate in a desired position with respect to the pad. The carrier includes means for providing a force to keep the substrate in contact with the pad, and also may include means for rotating, vibrating, or oscillating the substrate. During polishing, a slurry having both chemical and abrasive agents is supplied to the interface between the substrate and the pad, to enhance the rate at which material is removed from the substrate. The chemical agents included in the slurry are generally chosen to be reactive towards the material being removed by polishing. For example, when a metal material is being polished, the polishing slurry will be acidic in nature.
One problem associated with CMP is endpoint detection. Endpoint may be defined as the point at which the desired polishing operation is completed. When "endpoint" is attained, a number of different actions may be taken in response. For example, the entire polishing process may be terminated when endpoint is attained or the polishing conditions may be changed as the polishing process continues, with another polishing operation, to polish an underlying film. It can be seen that a substrate containing a stack of films to be polished, may include a number of discrete polishing operations, each of which includes an associated "endpoint".
Depending on the chemical mechanical polishing operation being performed, "endpoint" may signify different events. For example, when polishing a raw substrate, the "endpoint" condition may be attained when a certain predetermined substrate thickness has been removed. The same is true for a layer or film which is being partially removed. When a film is being completely removed from a substrate, "endpoint" is attained upon complete removal of the film. When CMP is used to planarize a substrate by removing portions of a film which extend above non-planar underlying features, "endpoint" is achieved when the surface is essentially planar. Generally speaking, an "endpoint" condition is attained after a predictable amount of material has been removed from the surface. It is therefore necessary to accurately detect when endpoint is achieved so that the polishing operation may be quickly terminated or otherwise adjusted at that point. Because the substrate is polished face-down and the polishing surface is generally contiguous with the polishing pad, a process monitor cannot easily be used to view the progress of the polishing operation, especially by directly monitoring the surface being polished. As such, it is difficult to attempt to use such a monitor to determine the polishing "endpoint".
Variations in the polishing conditions also impede an accurate determination of the polishing endpoint. For example, variations in the slurry composition, pad condition, relative speed between the pad and the substrate, the material being polished, and the load of the substrate on the pad, cause variations in the material removal rate. These variations in the material removal rate cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot reliably be estimated merely as a function of polishing time.
A common application of CMP is to partially or completely remove a deposited metal material from a substrate by polishing. One such example is to planarize a substrate surface using damascene technology. In damascene technology, trenches, grooves or other openings may be formed within a subjacent layer such as a dielectric film formed over a substrate. Next, a bulk deposited layer, generally a conductive material such as metal, is formed over the upper surface of the subjacent layer and within the openings which extend down into the subjacent layer. One aspect of CMP is to remove the bulk of the deposited metal layer from over the upper surface of the subjacent layer, leaving areas of the metal layer only in the openings formed within the subjacent layer. In this manner, a wiring pattern is produced. It can be understood that it is desirable to terminate the polishing operation when endpoint is achieved, i.e. when the bulk of the deposited metal layer is removed from over the upper surface of the subjacent layer, but remains within the openings so that the remaining portions of the metal layer form a substantially planar surface with the upper surface of the subjacent layer.
One general approach to predicting the polishing endpoint is to remove the substrate from the polishing surface and measure the thickness of the substrate or the film being removed by polishing. By periodically removing the substrate from the polishing apparatus and measuring its thickness, the quantity of material being removed from the substrate may be determined. As such, a linear approximation of the material removal rate may be used to determine the polishing endpoint. This technique is time consuming, however, and does not account for sudden changes in the removal rate that may occur between measurement intervals, or for other variations in the material removal rate as discussed above.
Several other non-invasive techniques for endpoint detection are known. These techniques generally fall into two categories: those which require access to the surface of the substrate being polished, and those which determine the polishing endpoint by determining changes in the operating conditions of the polishing apparatus.
Techniques included within first category typically require real-time access to at least a portion of the substrate surface being polished, such as by sliding a portion of the substrate over the edge of the polishing pad and simultaneously analyzing the exposed portion of the substrate. For example, where polishing is used to remove the bulk of a conductive film such as a metal, and to form metal lines embedded within trenches formed in a subjacent dielectric layer as in the planarization example discussed above, the overall or composite reflectivity of the surface being polished changes as the metal film is removed and the dielectric layer is exposed. By monitoring the reflectivity of the polished surface or the wavelength of light reflected from the surface, the polishing endpoint can be detected as the reflectivity changes when the dielectric layer is exposed. However, this technique does not provide a way of determining the polishing endpoint unless an underlying layer such as the dielectric, is exposed during polishing and has a reflectivity which varies from the film being polished. Additionally, it is somewhat erratic in predicting the polishing endpoint unless all of the underlying surface of a different reflectivity, is simultaneously exposed. Furthermore, the detection apparatus is delicate and subject to frequent breakdown caused by the exposure of the measuring or detecting apparatus to the polishing slurry.
Another technique included within first category involves projecting a laser beam through an opening formed in the polishing pad, and onto the surface being polished. This technique is not favored because of the difficulty associated with projecting a laser through an opening which must be formed in an otherwise contiguous, rotating polishing pad. Additionally, the window, through which the laser beam is projected, must be kept clean. This is quite difficult to do, especially with some commonly used polishing slurries.
Techniques for determining the polishing endpoint included within is the second category, generally do so by monitoring various operating conditions of the polishing apparatus and indicating an endpoint condition when one or more of the operating conditions abruptly changes. An example of such an operating condition is the coefficient of friction at the interface of the polishing pad and the substrate. When a metal layer is being polished to expose an underlying dielectric layer, for example, the coefficient of friction will change when the dielectric layer is exposed. As the coefficient of friction changes, the torque necessary to provide the desired polishing pad speed also changes. By monitoring this change such as by monitoring the polishing motor current, endpoint may be detected. However, the coefficient of friction is a function of the slurry composition, the pad condition, the load of the substrate on the pad, and the surface condition of the substrate. In addition, the pad condition and the slurry composition at the pad-substrate interface changes as the substrate is being polished. Moreover, electrical noise may distort the characteristic being measured. Such effects may mask the exposure of the underlying dielectric layer (and removal of the bulk of the metal film), and they may prematurely endpoint the polishing operation. Additionally, using this method, the endpoint detection will work only if polishing is used to expose an underlying material having a frictional attribute different than that of the material being removed.
Another technique for determining endpoint included within the second category involves monitoring the power input to one or more of the polishing motors, such as the motor which rotates the polishing pad or a motor which may be used to rotate the substrate being polished. Using this technique, a determination that endpoint has been achieved, may be made when a pre-determined power sum is reached. Like the other techniques within the second category of endpointing techniques, this method also does not directly monitor physical activity occurring on the surface being polished during the polishing operation.
Therefore, none of the available endpointing techniques described above, detects endpoint by directly monitoring the film being removed, or other physical changes occurring on the surface being polished, without interrupting the polishing process. As such, none of the known techniques for determining endpoint, do so by actually sampling the surface during the polishing operation, and detecting that the bulk portion of the film being polished, is physically removed from the surface. It can be understood, then, that such a method and an apparatus for performing the same, are desirable in the art of CMP.
For the aspect of CMP directed to completely removing a metal layer or to forming conductive lines within trenches or the like using damascene techniques, endpoint is attained when the bulk of the metal material is removed from over the upper surface of the subjacent layer, but remains within the trenches to form a planar surface. In damascene applications, the metal film which remains within the trenches, produces a wiring pattern within the planar surface. At this point, it is desirable to terminate or otherwise adjust the polishing operation. Since the polishing slurry used to polish a metal material is acidic in nature and is therefore reactive towards the metal layer being polished, the polishing process produces reaction products such as hydrogen vapor. As such, when endpoint is achieved and the bulk of the metal layer is removed from over the substrate surface, the concentration of hydrogen vapor within the product stream, drops.
It can be seen that there is a need for an endpoint detection apparatus which can be used to detect endpoint at this point in order to terminate or otherwise adjust the polishing operation to avoid further undesired polishing.